osd: set r only if succeed in FillInVerifyExtent

osd: set r only if succeed in FillInVerifyExtent

When read failed, ret can be taken as data len in FillInVerifyExtent, which should be avoided.
It may cause errors in crc repair or retry read because of the data len. In my case, we use FillInVerifyExtent for EC read,
when meet -EIO，we will try crc repair, which need read data from other shard accrding to data len.
And I meet assert in ECBackend.cc (loc: line 2288 ceph_assert(range.first != range.second) ), But it seems master branch not support EC crc repair.
In shot, when reuse the readop may cause unpredictable error.

Fixes: https://tracker.ceph.com/issues/51115
Signed-off-by: yanqiang-ux <yanqiang_ux@163.com>